The invention relates to a use of a sampling apparatus shaped with the aim of taking samples isokinetically from a mass flow in the form of a two- or multi-phase fluid in a pipeline assigned i.a. electronic measuring apparatus. The sampling apparatus is meant to be coupled continuously to the pipeline, either at the seabed or at the well head on a platform.
Prior art techniques comprise test separators separating the fractions (oil, gas, water, sand, condensate) of the multi-phase fluid. The amount of production for each fraction per unit of time is measured. This must be done regularly at each well.
Such test separators are heavy, and they are expensive in purchase and use.
In order to reduce the costs and simultanelously to enable a continuous testing of the wells in respect of the composition of the multi-phase fluid, it has been of great interest to find new technological solutions. Mass flow measuring has resulted in the use of electronic fraction measuring apparatus and metering appliances for measuring temperature and pressure in a multi-phase fluid flowing in a pipeline. Mass flow or fraction metering appliances enable electronic metering of the amount of gas, oil, sand and water flowing past a certain pipe cross section per unit of time. This electronic multi-phase measuring has experienced great confidence. Mass flow or fraction meters exist in various embodiments and designs, and may be based on the measurement of electrical voltages in the various fractions.
However, the electronic fraction meters that have been in use up to now have not been sufficiently accurate. They are adapted to transmit signals to a computer displaying the measuring results, which can differ rather significantly from the actual values. The corresponding measuring values achieved by means of the test separator may possibly be used as prescribed values when the electronic fraction meters are calibrated, but one desires to eliminate the test separators because of their weight, high price and substantial operational costs.
The object of the present invention has been to eliminate the use of test separators and, thus, to relieve the platform, obtaining significant cost savings; simultaneously enabling the provision, at all times, of accurate measuring results well suited as prescribed values for the calibration of said electronic mass flow/fraction meters.
By means of a sampling apparatus, known per se, possibly modified, adapted for isokinetic sampling, a probe is inserted into the mass flow within a pipeline, said probe, preferably, having two oppositely directed orifice plates, in order to bring out one single sample or a sample taken in co-current and a sample taken counter-currently. The sample taken isokinetically, is analyzed, e.g., in a spectrograph. The measuring values obtained are very close to the actual values and are, thus, well suited as prescribed values constituting a base for calibrating calibratable electronic mass flow/fraction meters. Corrections may be placed in the software of the processor unit of the electronic measuring instruments. The sampling apparatus which, e.g., may be of the kind as described and shown in Norwegian patent specification No. 173,468, enables isokinetic sampling. This apparatus comprises an actuator coupled to a check valve which, in turn, is coupled to the fluid-carrying pipeline through a through hole in the pipe wall. When the check valve occupies an open position, the actuator can place a probe into said hole, laterally of the longitudinal direction of the pipeline. The probe may have one or two diametrally opposite orifice plates, which can be moved across substantially the entire inner span of the pipeline, in order to take out fluid samples from at least the main part of the cross-sectional area of the fluid flow.
If the apparatus is placed at the seabed, this/these sample(s) may be delivered on board the platform through a pipeline for analysis.
The sampling apparatus is adapted to be permanently mounted to the pipeline on the seabed or on the well head of a platform, and makes a test separator completely superfluous.
A sampling apparatus used in accordance with the present invention serves primarily to provide accurate measuring results associated with the amounts, expressed in percentages, of the respective fractions for use as prescribed values for the calibration of the fraction meters, but enables secondarily the achievement of other measuring results, such as pressure, temperature, etc.
In accordance with the invention, there has been provided an independent measuring technique based on the use of a sampling apparatus in order to improve measuring results in quality, and to take samples. The sampling apparatus may be placed upstream or downstream in relation to the calibratable electronic fraction meters.
It may be an advantage to use the sampling apparatus in combination with a per se known, possibly modified mixing apparatus e.g., of the kind shown and described in Norwegian patent specification No. 174,015. Such a mixing apparatus would be mounted to said pipeline upstream in relation to the sampling apparatus and comprises a throttle body which is displaceable laterally of the longitudinal direction of the pipeline and which in operative position registers with the inner cross-sectional area of pipeline. The throttle body has converging channels, tapering in the direction of flow, and the longitudinal axes thereof meet in an imaginary, so called "focal point" (crossing point), and serve to mix the various fractions of the multi-phase fluid. Without such a mixing, one could risk that a liquid jacket immediately adjacent the inner wall surface of the pipeline would not be included in the sample. This mixing apparatus, which possibly could be deleted, has hydraulic actuators; the control lines thereof may be included in a so called umbilical, or they may be operated from, e.g., a remote control vehicle.
Isokinetic sampling gives accurate samples for the determination of produced amounts of oil, of gas, of water, of formation sand and of condensate per unit of time, as well as temperature and pressure at the measuring point on the pipeline. If the sampling apparatus, possibly together with the mixing apparatus, are placed on the well head on the platform, the apparatus may be controlled from a control room and operated manually.